I don't think you need a dash kit. I just installed a U310 in my 06 Subaru WRX, Which Im pretty sure has the same dash set up, and I didn't need a dash kit. Pretty sure all 02-07 Impreza's are the same. It fits very nicely in the Subaru double din opening.
You'll want to get a wirng harness to make it easiler and keep your factory plug intact.